A polymer electrolyte fuel cell (hereinafter simply referred to as “fuel cell”) generally includes a membrane electrode assembly having electrodes provided on both surfaces of a proton-conductive electrolyte membrane, as a power generation element. A catalyst for accelerating the fuel cell reaction is supported on the electrode. In a fuel cell system mounted on, for example, a fuel cell vehicle, continuous operation of the fuel cell at a high voltage causes formation of an oxide layer on the surface of the catalyst and is likely to reversibly deteriorate the catalytic performance. In such cases, a known technique recovers the catalyst by lowering the potential of the air electrode of the fuel cell (for example, Patent Literature 1).